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• Ditches that carry water from undisturbed areas will receNe no runoff from a 10-year, 24-hour precipita- <br />tion event using the approved hydrologic modeling parameters For use at Trapper. These ditches have <br />been designed at a flow depth of 0.25 feet to handle any snowmelt or spring(seep runoff. <br />All major diversions will be built with side slopes of 3H:1V and have vertical slopes that are non-erosive. <br />In the event excessive erosion occurs, appropriate repairs will be implemented. All diversions will have a <br />minimum freeboard of 0.3 feet. <br />The locations of all major diversions will be within 100 feet of those shown on Map M51, Drainage and <br />Sediment Control Plan. However, unless Division approval is received, the location of the proposed point <br />of discharge won't be changed 'rf it results in a functional change to the overall system design. Mod'rfica- <br />tions exceeding 100 feet lateraly or other such major modification to major diversion ditches will be sub- <br />mitted to the Division for approval. <br />Most temporary diversion and drainage ditches which occur at Trapper Mine are constructed on areas <br />which have had topsoil removed, or on spoils prior to topsoil replacement. Erosion within these ditch <br />channels is irrelevant, since the site will either be mined through, recontoured, and drain relatively small <br />areas. Since the flow carried by these structures is directed to sediment control dams, additional contri- <br />butions of suspended solids to streamflow and runoff outside the permtt area are minimized. These <br />diversions and drainages are constructed with dozers, graders, or other suitable equipment. <br />• Normally the temporary diversion and drainage d'AChes constructed at Trapper Mine have a drainage <br />area of ten acres or less. In these cases, the temporary structures will approximate the dimensions illus- <br />trated in Figure 4.8-1. Table 4.8-4 shows the methodology used to calculate peak slows and the resulting <br />ditch configuration in Figure 4.8-1. Table 4.8-5 shows estimated peak flows from areas with slopes of 8% <br />to 14% with various 10-acre configurations. The table demonstrates that the design criteria in Figure 4.8- <br />1 can accommodate peak flows from a variety of drainage area shapes. <br />• P2 3 -_---- -----_-- <br />_, ~ <br />a-1~a ;': ;?FVISIO F~P~'R~dt~D ~I <br />DATE ~ ~ ~ <br />- ~~ <br />i --- <br />